1. Field of the Invention
The present invention relates to a method and an installation for the dry transformation of a material structure of semifinished products.
2. Description of Related Art
For the improvement of material properties of metallic component parts, it is known that one may influence their material structure using heat treatment methods. Steels are particularly suitable for such treatment methods, in addition to a great multitude of metals, and of the steels, for instance, 100Cr6 reacts well to treatment using such bainitic structure tempering methods.
With regard to 100Cr6, heating of the material is first carried out to a temperature of approximately 850° C., for example, so that a so-called austenitic structure is formed in the material. Thereafter, the component parts thus heated have to be quenched very rapidly to the bainitic structure tempering temperature in their entire body temperature, that is, also on the inside of the component parts. A temperature range of ca. 220° C. is used for this, at which the so-called bainitic structure comes about. However, this temperature is only slightly above the so-called martensite start temperature, to which the work pieces absolutely must not cool off during the structural transformation process, since this would result in massive interference in the desired and particularly advantageous bainitic structure.
Other interferences in the formation of the bainitic structure may be brought on by cooling the component parts too slowly. In this connection, the pearlite structural region should particularly be mentioned. The pearlite structural region sets in approximately between 730° C. and 470° C. in response to a longer residence of the material in this temperature range. A further disturbance is represented by the so-called continuous bainitic range, whose upper temperature range overlaps with the lower temperature range for the formation of the pearlite structure. Its lower temperature range reaches down to the vicinity of the bainitizing range, depending on the residence duration of the material.
In order to avoid the formation of such undesired structures in the parts that are to be treated, a cooling time for the entire part, that is, both outside and inside in the core, of 35 seconds to 40 seconds is regarded as being necessary.
To overcome the disadvantages known from salt bath cooling methods that have been used up to now, such as harmfulness to the environment, purity problems in the salt bath, purity problems in the parts and cost intensity, so-called dry austempering methods have been developed. In these methods, the parts are quenched on the inside of a quenching chamber, using a temperature-controlled gas. To be able to dissipate the enormous energy liberated in this process, an appropriate gas flow is applied to the inside of the quenching chamber.
For the purpose of regulating the temperature of this gas stream, published German patent document DE 100 44 362 proposes, for instance, a variation of an effectively overflowed surface of a heat exchanger that cools the gas. In another method, an active control of the gas temperature is proposed, using two gas flow channels arranged in parallel, one channel being cooled and the other heated. The flow proportion of the hot and the cold channel is supposed to be appropriately adjusted via valves, in this instance, to regulate the gas temperature.
However, both these methods are encumbered by the problem that, depending on the response of the controlled system, the gas temperature oscillates about the setpoint temperature (bainitic structure tempering temperature), at least temporarily. Therefore, it may not be excluded that the gas temperature briefly falls below the martensite start temperature, and thereby at least endangers, if not even prevents, the structural development of bainite, for instance, in the component parts. This happens because the edge regions of a component part very rapidly take up the gas temperature, especially in thin-walled places, at corners or at courses of thread.